Removing a heavy metal from a combustion gas

Abstract

This invention is directed to a process and chemical composition effective in removing heavy metal from combustion gas. The process and chemical composition are particularly effective in removing undesirable heavy metal from combustion gases that are formed from the combustion of coal. The chemical composition that is used is an aqueous composition of at least one alkali metal sulfide, and at least one alkali metal or alkaline earth metal buffer in amounts that are effective in removing at least a majority of the heavy metal from the combustion gas.

1 . A process for removing a heavy metal from a combustion gas, comprising:
 providing a combustion gas containing a heavy metal that is desired to be removed from the combustion gas; and   contacting the combustion gas with an aqueous composition of at least one alkali metal sulfide, and at least one alkali metal or alkaline earth metal buffer to remove at least a majority of the heavy metal from the combustion gas.   
     
     
         2 . The process of  claim 1 , wherein the combustion gas is formed from the combustion of coal. 
     
     
         3 . The process of  claim 1 , wherein the heavy metal that is desired to be removed is selected from the group consisting of zinc, cadmium and mercury. 
     
     
         4 . The process of  claim 3 , wherein the heavy metal that is desired to be removed is mercury. 
     
     
         5 . The process of  claim 1 , wherein the at least one alkali metal sulfide is a compound comprising a S −2  ion or a HS −  ion. 
     
     
         6 . The process of  claim 1 , wherein the at least one alkali metal sulfide is a mono-sulfide or a poly-sulfide. 
     
     
         7 . The process of  claim 1 , wherein the alkali metal of the at least one alkali metal sulfide is sodium or potassium. 
     
     
         8 . The process of  claim 7 , wherein the alkali metal of the at least one alkali metal sulfide is sodium. 
     
     
         9 . The process of  claim 1 , wherein the at least one alkali metal or alkaline earth metal buffer is a carbonate, hydroxide or phosphate compound that includes at least one element selected from the Group 1 and Group 2 elements of the periodic table. 
     
     
         10 . The process of  claim 1 , wherein the at least one alkali metal or alkaline earth metal buffer is a carbonate, hydroxide or phosphate compound that includes sodium or potassium. 
     
     
         11 . The process of  claim 1 , wherein the at least one alkali metal or alkaline earth metal buffer is a carbonate, hydroxide or phosphate compound that includes magnesium or calcium. 
     
     
         12 . The process of  claim 1 , wherein the aqueous composition comprises an alkaline earth metal buffer and a phosphate buffer different from the alkaline earth metal buffer. 
     
     
         13 . The process of  claim 12 , wherein the phosphate buffer contains one or more of magnesium and calcium. 
     
     
         14 . The process of  claim 14 , wherein the aqueous composition comprises sodium hydrosulfide, calcium carbonate and triple superphosphate. 
     
     
         15 . An aqueous composition effective in removing heavy metal from combustion gas, comprising:
 at least one alkali metal selected from the Group 1 elements of the periodic table in an amount effective for removing the heavy metal from the combustion gas; and   at least one alkali metal buffer or alkaline earth metal buffer at a concentration effective at buffering the aqueous composition at a pH of from 5 to 11.   
     
     
         16 . The aqueous composition of  claim 15 , wherein the at least one alkali metal sulfide is a compound comprising a S −2  ion or a HS −  ion. 
     
     
         17 . The aqueous composition of  claim 16 , wherein the alkali metal of the at least one alkali metal sulfide is sodium or potassium. 
     
     
         18 . The aqueous composition of  claim 15 , wherein the at least one alkali metal or alkaline earth metal buffer is a carbonate, hydroxide or phosphate compound that includes at least one element selected from the Group 1 and Group 2 elements of the periodic table. 
     
     
         19 . The aqueous composition of  claim 15 , wherein the at least one alkali metal or alkaline earth metal buffer is a carbonate, hydroxide or phosphate compound that includes magnesium or calcium. 
     
     
         20 . The aqueous composition of  claim 15 , wherein the aqueous composition comprises an alkaline earth metal buffer and a phosphate buffer different from the alkaline earth metal buffer.